This invention generally relates to fiber optic sensors, and more specifically to using optical amplifiers to improve Angle Random Walk noise in fiber optic sensors by increasing the optical power at a detector of the fiber optic sensor.
The angle random walk noise (ARW) of a fiber optic sensor, such as a fiber optic current sensor or fiber optic gyroscope, is comprised of noise arising from the transimpedance amplifier feedback resistor (thermal noise), shot noise related to the detector current, and flicker (1/f) and relative intensity noise (RIN) intrinsic in the light impinging on the photodetector. The first is independent of the light power, the second can be shown to be proportional to the square root of optical power, and the last two (flicker, RIN) are proportional to the optical power. What this means is that for any given sensor configuration (coil length, diameter, light source power, etc.), the effects of thermal and shot noise will decrease with increasing optical power, while the flicker noise and RIN effects cannot be reduced by increasing the optical power. For convenience, RIN will be used hereafter to refer to both flicker noise and true RIN effects.
The noise components in the sensor output can be represented as:
N=k(A2P2+B2P+C2)1/2xe2x80x83xe2x80x83Eq. (1) 
where
A is the RIN noise;
B is the shot noise;
C is the thermal noise;
P is the optical power at the detector; and
k is a proportionality constant.
The output signal of an open-loop fiber optic gyroscope (FOG) for small rotation rates is
S=mxcfx86xcexa9P,xe2x80x83xe2x80x83Eq. (2) 
where
xcexa9 is the angular rotation rate;
m is a proportionality constant;
xcfx86=4xcfx80RL/c xcex is the Sagnac or optical scale factor;
R is the radius of the equivalent coil; and
L is the coil length.
Thus, the signal-to-noise ratio is:
S/N=mxcfx86xcexa9/(k(A2+B2/P+C2/P2)1/2)xe2x80x83xe2x80x83Eq. (3) 
and the shot and thermal noise components decrease as the optical power is increased. However, the contribution of RIN to the signal-to-noise is unchanged and the performance of a FOG with conventional signal processing ultimately becomes limited by RIN. Usually the thermal noise component can be ignored.
FIG. 5 illustrates the dependence of the individual noise components as a function of detected optical power. (Lefevre, xe2x80x9cThe Fiber-Optic Gyroscopexe2x80x9d, Artech House, Boston 1993). The inverse of the signal-to-noise ratio or relative noise is shown on the ordinate and is called the xe2x80x9cAngle Random Walkxe2x80x9d (ARW). This can be interpreted as the minimum detectable rotation signal when normalized to a one Hz bandwidth. In FIG. 5, xe2x80x9csource noisexe2x80x9d represents the RIN.
The reduction of RIN is addressed in xe2x80x9cApparatus and Method for Electronic RIN Reduction in Fiber-Optic Sensorsxe2x80x9d, by Bennett, U.S. patent application Ser. No. 09/481,159. Here we look at the shot noise for situations where either the RIN had been reduced to the point where shot noise is dominant, or situations where the detected power is low enough so that the shot noise can be considered the dominant component. (In the context of FIG. 5, a region where shot noise is dominant exists between optical powers of 10xe2x88x925 and 10xe2x88x924 watts.). To simplify the discussion, we will assume that shot noise is the only noise present.
Shot noise arises from the interaction of individual photons in the light beam incident on the detector with the physical matter of the detector itself. The effect is quantized in nature so that some number (which could be fractional) of electrons is liberated for each photon impinging on the detector in a spectral regime where the detector exhibits a photoelectric effect.
From this discussion, it can be seen that the concept of detected signal-to-noise differs fundamentally from that employed in radio reception systems. In those systems, the noise power is not a function of the input signal, and signal-to noise ratio increases linearly with signal power. Even in optical telecommunications systems, where the thermal, shot and RIN noise components are present, RIN noise can be reduced by limiting the bandwidth of the optical signal at the input to the photodetector to that required by the modulation bandwidth by optical bandlimiting. In a fiber optic sensor, this is not possible, as a broad optical bandwidth (usually greater than several nanometers) is necessary to overcome certain deleterious optical effects, such as polarization cross-coupling and Rayleigh scattering.
The component of ARW due to shot noise can be reduced, by increasing the optical power at the detector. This is done in existing art by increasing the optical power emitted by the optical source, which may be a superluminescent diode (SLD), a semiconductor laser operated below threshold, a laser modified as in xe2x80x9cBroadening the Linewidth of a Semiconductor Laserxe2x80x9d by Dyott, U.S. patent application Ser. No. 09/568,371 which is incorporated herein be reference, or a rare-earth-doped fiber amplifier. This is usually done in the so called xe2x80x9cminimum configurationxe2x80x9d (MC) fiber optic gyroscope, but may also be done in the xe2x80x9cReduced Minimum Configurationxe2x80x9d (RMC) device (xe2x80x9cMonomode Optical Fiber Ring Interferometric Device with Semiconductor Diode as Light Energy Emission Reception/Amplification Meansxe2x80x9d, U.S. Pat. No. 4,842,409 to Arditty, et al. and xe2x80x9cReduced Minimum Configuration Interferometric Fiber Optic Gyroscope with Simplified Signal Processing Electronicsxe2x80x9d, U.S. patent application Ser. No. 09/459,438 by Emge et al.). High power optical sources are known and used, but the cost of these devices increases substantially as the optical power is increased, and the reliability also is reduced due to various damage mechanisms that arise at high optical power densities. It is also known that the fiber exhibits a number of non-reciprocal effects and non-linearities at high optical powers, which can lead to degradation of other desired properties such as bias stability and scale factor.
According to the systems and methods disclosed herein, a shot noise component of Angle Random Walk noise in a fiber optic sensor may be reduced by providing an optical amplifier between a first coupler receiving a sensor signal from a sensing coil of the sensor and a photodetector receiving the sensor signal from the first coupler.
Another embodiment may further comprise providing a second detector on a free leg of the first coupler to receive a source sample from an optical source of the fiber optic sensor; delaying the source sample to provide a delayed source sample coinciding with the sensor signal; modulating the delayed source sample to provide a modulated source sample; and comparing the modulated source sample with the sensor signal so as to subtract Relative Intensity Noise.
Another embodiment may further comprise providing an isolator between the first coupler and the optical amplifier to suppress back facet emissions of the optical amplifier reaching the first coupler.
Another embodiment may further comprise: providing an additional coupler between the optical amplifier and the isolator; providing a third detector on a first leg of the additional coupler to receive the back facet emissions from the optical amplifier; and subtracting the back facet emissions received at the third detector from the sensor signal received at the photodetector.
A still further embodiment further comprises providing a polarizer immediately adjacent one or more of the detectors to preclude emissions in an unwanted polarization from reaching the detector to which the polarizer is adjacent.